Recent content by dreens

Thanks all. I really like that article series by Phil Danielson. It tickles me every time he refers to us, the readers, as “vacuum practitioners” or “practitioners of high vacuum”. Seems to happen at least once near the beginning of all his articles, and I wouldn’t want it any other way. It’s...

Hi there, I have a small, few liter UHV chamber that frequently needs to be vented and rebaked. I want to shorten my bake time, so I’ve obtained a glove box to allow me to vent, work and close back up under inert gas. i’m curious if people have a preference between Argon and dry Nitrogen. I...

A quantum theory of gravity would be unlikely to mess with the ISL at long distances. Generally speaking, quantum theories behave classically at long enough length-scales and low enough energy scales. However, it would be much more likely to mess with ISL at short distances. With a quick...

Perhaps its useful to think about this photon energy question in a simpler setting. Consider photons emitted from someone speeding away from you. They arrive at a different frequency, which can be calculated based on relativistic doppler formulas. However, the intensity of the light is also...

Okay so I finished Garrity’s E&M for mathematicians. Very fun! I now know how to think about and use wedge products and forms, integrate them on n-d manifolds, define connections for vector bundles over a manifold, etc. Or at least I’ve been thoroughly reminded, since I did sleep through a...

Ah of course the orientation. Hopefully we’ve finally ironed out each other’s mistakes ;-) The lack of isopotry in direction is so odd in the 3-torus. Would definitely be a real surprise if that was our universe. Although not as big a surprise as if we were a 3D version of a Klein bottle-...

Thanks for the WMAP link, great stuff! Touché. I agree with your “(?)” though- I think it’s only 1 or 2 paramaters. The donut doesn’t care where you think of the edge as being, so that knocks out 3. The dataset is probably only 2D so that knocks out another. Then they could have fixed the...

Agreed. That’s a more obvious way to get a finite, uncurved, 3D space. Especially if you think of a 3-torus as a cube with opposite sides identified, so that if you go through the right you appear on the left, etc. Does anyone know how well this possibility is excluded by astrophysical...

I like you’re thinking here. There are ways that a universe obeying general relativity could exist and be finite. I don’t think a universe with a sharp edge is viable mathematically... the curvature tensor would become undefined at the edge. But one of the simplest possibilities would be S^3...

Wow everyone, thanks for the awesome responses. I'm going to go grab this one from the library right now. It sounds perfect since I have a math theory degree with coursework in algebraic topology and have missed doing math lately. (I'm a physics PHD now, but not theoretical, I like building...

So General Relativity explains the force of gravity as mass/energy induced curvature of spacetime. This correctly predicts gravitational time distortion, nonlinear geodesics and gravitational lensing, the anomalous precession of planetary orbits, the schwarzchild metric, and so on. Could the...

Name recognition and alumni networks can be really powerful. Maybe more-so than grades? But just get good grades either way ;-). I chose a half-pay big name over a full ride at a public ivy. My family did suffer somewhat, but they were strongly supportive of the decision. The name recognition...

Wow that's an interesting paper. I'm impressed that this can be solved, even for a pair of particles as in the appendix. I don't have the heart to look at it closely enough to try and figure out why the math is giving you the wrong sign. Not that it's worth more than two cents, but I'll bet...

I agree with your intuition. Motion should be in -y direction. I don't think you can solve Navier Stokes very easily by hand in this geometry... what calculations are you doing?

It's a bit hard to picture this without understanding more about what you are trying to lock. It seems you have a shaft that is constrained only to rotate, and you want to use a solenoid actuator to hold this shaft in place... your idea is to have the pin that the solenoid pushes interrupt the...